FIG. 14 is a diagram depicting a configuration of a conventional antenna demultiplexer. A conventional antenna demultiplexer 81 is configured such that a transmission filter 82 is connected between a common terminal Ant and a transmission terminal Tx, and a reception filter 83 is connected between the common terminal Ant and a reception terminal Rx. In the case in which the antenna demultiplexer 81 is used in, for example, a mobile phone, the common terminal Ant is connected to an antenna, the transmission terminal Tx is connected to a transmission circuit, and the reception terminal Rx is connected to a reception circuit.
The passband of the transmission filter 82 and the passband of the reception filter 83 are different, and therefore when a transmission signal that has been input to the transmission terminal Tx passes through the transmission filter 82, such transmission signal is output from the common terminal Ant to the antenna without flowing to the reception filter 83 side. The transmission signal is then converted to radio waves and emitted by the antenna. On the other hand, a reception signal that has been received by the antenna passes through the reception filter 83, and is input from the reception terminal Rx to the reception circuit. This is the flow of signals in an ideal antenna demultiplexer.
However, in an actual antenna demultiplexer, not all of the transmission signals that have been output from the transmission filter 82 are output from the common terminal Ant to the antenna, but rather a portion of such transmission signals flow to the reception filter 83 and arrive at the reception terminal Rx. For this reason, reception signals that are input from the reception terminal Rx to the reception circuit include noise, and as a result, there is the problem that reception performance deteriorates.
Here, transmission-reception isolation (hereinafter, simply called isolation) is used as a value indicating the degree to which transmission signals that have been input from the transmission circuit to the transmission terminal Tx, leaked to the reception filter 83 side, and arrived at the reception terminal Rx, can be suppressed. Normally, the isolation of an antenna demultiplexer is approximately 50 dB. This isolation value of 50 dB is not sufficient for satisfying the performance required for antenna demultiplexers.
Conventionally, a filter has been added within the reception circuit in order to prevent such deterioration in reception performance, but this has led to an increase in the number of parts, cost, and complexity. Accordingly, improving isolation in antenna demultiplexers has become an industry-wide issue in mobile phone designing.
In view of this, there has been a proposal for a structure for removing unnecessary electromagnetic coupling in antenna demultiplexers, which is one cause for the leakage of transmission signals from the transmission circuit to the reception circuit (e.g., see the below Patent document 1 and Patent document 2). Patent document 1 discloses a configuration in which a shield electrode is provided in order to remove unnecessary electromagnetic coupling. Patent document 2 discloses an example in which unnecessary electromagnetic coupling is prevented by an innovation in the connection with the ground terminal of the antenna demultiplexer package.    Patent document 1: Japanese Laid-open Patent Publication JP 2006-60747A    Patent document 2: Japanese Laid-open Patent Publication JP 2002-76829A
However, the configurations disclosed in the above Patent documents 1 and 2 merely remove unnecessary electromagnetic coupling, and do not fundamentally reduce or eliminate leaked signals from the transmission filter, or improve isolation. In order to fundamentally suppress such leaked signals, it is necessary to raise the degree of suppression in the filter suppression band.